1. Technical Field
This invention relates to the safety mechanism of lithium secondary batteries such as lithium ion secondary batteries.
2. Background Art
The recent progress of portable equipment is outstanding, and high energy batteries including lithium ion secondary batteries make great contribution as one factor to the motive power therefor. The current market of lithium ion secondary batteries has grown beyond annual 300 billion yen. It is forecast that a variety of portable equipment will be developed in the future, with an advance of compliant battery manufacturing technology being demanded.
The lithium ion secondary batteries are generally constructed by a positive electrode, a liquid or solid electrolyte layer and a negative electrode. The positive or negative electrode material is prepared by mixing a positive or negative electrode active material with a conductive agent and a binder and coating the mixture onto a current collector. The development trend of such lithium ion secondary batteries demands to increase the energy density of batteries, and the development of low-profile batteries is in progress as one solution.
One approach of manufacturing such low-profile lightweight batteries relates to a polymeric battery which is made thin by substituting a solid electrolyte for the electrolytic solution. This technology is known from, for example, U.S. Pat. No. 5,418,091. As a result of improvements being recently made in battery properties, the battery characteristics is now improved to a far superior level to the early stage when the technology was disclosed.
Batteries using such solid electrolytes include various forms and are generally classified into the following three types:    type (1) using lithium ion conduction in a polymer as the electrolyte,    type (2) using lithium ion conduction in a plasticized polymer as the electrolyte, and    type (3) using lithium ion conduction in an organic solvent and a plasticized polymer as the electrolyte.
Of these, batteries belonging to type (3) obtained by mixing a solvent component, an organic polymer component and an electrolyte salt to form a gel or solid are in progress toward practical use because they exhibit characteristics comparable to the solution type batteries.
Typical of the method of manufacturing batteries of type (3) using gelled solid electrolytes are the battery manufacturing methods described, for example, in U.S. Pat. No. 5,296,318 and U.S. Pat. No. 5,418,091. These methods involve preparing a solid electrolyte medium from a solid-state polyvinylidene fluoride, joining it to positive and negative electrodes, extracting a plasticizer from the entire battery cell, and introducing an electrolytic solution to gel the entire cell.
Since the entire battery cell is gelled in this way, the liberated electrolytic solution is absent in the battery interior. The battery is thus deemed to take a completely different form from the prior art solution type batteries. According to the disclosure of U.S. Pat. No. 5,296,318 and U.S. Pat. No. 5,418,091, the battery characteristics are also superior.
When the solid-state gelled electrolyte is used, no problem arises on normal use, but outstanding problems arise in an abnormal state. Specifically, safety tests of batteries include an over-charging test, a nail penetration test simulating internal short-circuit (known as hard short-circuit), a heating test and the like. Of these tests, the internal short-circuit test is to forcedly induce internal short-circuits as by penetrating nail under fully charged conditions. As the battery capacity becomes larger, more short-circuiting currents flow. As a result, a rapid temperature rise occurs with the battery, and the battery itself undergoes thermal runaway. An exemplary countermeasure to this problem is to use a separator having a shutdown function. Since this measure relies on the shutdown response of the separator membrane, the battery taking this measure alone encounters a limit when subjected to a nail penetration test of inviting rapid short-circuits.
Another probable countermeasure is to increase the impedance of the battery interior. Since the charge/discharge characteristics of the battery are sacrificed, this measure, though employable in some particular applications, encounters a limit in an actual sense.
Meanwhile, lithium secondary batteries using a laminate type electrode structure are characterized by a high freedom of geometric design as compared with wound type lithium secondary batteries, enabling the construction of low profile, large area batteries.
The outermost layer of the electrode laminate is provided by a negative or positive electrode. In the event an electrode having an electrode active material coated on one surface of a current collector is used as the outermost layer, the outermost layer electrode can warp. Especially when the metal foil serving as the current collector is thinner than 30 μm and the electrode active material layer is thicker than 50 μm, the electrode suffers substantial warpage, imposing a serious problem to the manufacture of electrode structure. Also, the warpage of the outermost layer electrode can adversely affect the adhesion between electrodes and cause to degrade battery characteristics such as cycle performance.